Air batteries are capable of containing a higher proportion of the negative electrode active material in the battery case, in comparison with such as lithium secondary batteries. Therefore, they are capable of obtaining a higher discharge capacity, which means that size reduction and weight reduction of the batteries are easier. Moreover, the air batteries have a low environmental impact because they use oxygen as the positive electrode active material, which is free from constraints on resource availability and also is a clean material. For these reasons, it is expected that the air batteries will be widely used for such as batteries for electric vehicles, batteries for hybrid vehicles, and batteries for fuel cell vehicles.
Here, it is required that the positive electrode for the air battery have electrical conductivity, chemical stability, and capability of supplying oxygen-originating hydroxide ions. In view of this, a positive electrode for an air battery that has a catalyst layer formed by adding activated carbon and a catalyst to Teflon (registered trademark) powder (Patent Literature 1) and a positive electrode for an air battery that uses a carbon material that permeates gas but does not permeate liquid (Patent Literature 2) have been disclosed.
In the conventional positive electrode for an air battery as described in Patent Literature 1 above, the carbonaceous component existing on the surface has a small surface area, so the supply of hydroxide ions is inhibited by the components other than the carbonaceous component. In addition, the conventional positive electrodes for an air battery as disclosed in Patent Literatures 1 and 2 need to have communicating pores for passing oxygen gas along a thickness direction of the positive electrode, in order to supply oxygen that is necessary for supplying hydroxide ions. However, the communicating pores cause a greater amount of the electrolyte solution to evaporate therethrough, and consequently, the amount of the electrolyte solution decreases. This results in the problems such as the deterioration in battery output power. Another problem has been that, because the HOPG used in Patent Literature 2 above is a costly material, the manufacturing cost of the battery is higher.
In order to solve the foregoing problems, an expanded graphite sheet is expected to be used as the positive electrode for an air battery.